This invention relates generally to a multi-channel communication system and more particularly to a multi-channel differential pulse code modulation system.
Telephone systems in the United States and Europe have adopted standards for voice transmission by pulse code modulation (PCM) techniques. In such systems, the source signal is periodically sampled to form a sequence of analog samples which are applied to an analog-to-digital converter. The converter generates a representative digital code word for each sample. The sequence of digital code words is applied to a digital transmission facility. At the receiving end of the transmission system, the digital code words are converted to a sequence of analog samples corresponding to the original analog samples by applying them to a digital-to-analog converter. Where there are a plurality of analog signal sources, they may be sequentially and periodically sampled to form a time division multiplex of analog samples which are applied to a single analog-to-digital converter to form a time division multiplex digital signal for transmission. At the receiving end, the time division multiplex digital signal is applied to a single digital-to-analog converter whose output is applied to an analog demultiplexer which serves to sequentially and periodically distribute the reconstructed analog signal samples to corresponding signal sinks.
Another method of converting an analog signal to digital form is by delta modulation. The analog source signal is periodically compared to an approximating signal representing the prior signal value stored in a local accumulator. The comparison generates logic 1 or logic 0 according to whether the source signal is greater or less than the accumulator signal. After each comparison, the accumulator signal is incremented by one step positively or negatively according to whether the comparison yielded logic 1 or logic 0, thereby providing an updated approximation signal for the next comparison. By this mechanism, the local accumulator signal is forced to be a stepped approximation to the source signal. The digital signal generated by the comparison is transmitted, and received by a receiving accumulator identical to the transmitter accumulator. Therefore, the output signal of the receiving accumulator is also a stepped approximation of the source signal and provides a reconstruction of the analog signal.
The delta modulation process generates one code bit for each comparison, whereas PCM generates one multibit code word for each sample. Therefore, at equal bit rates, the comparison rate for delta modulation is many times greater than the sampling rate for PCM.
A delta modulation process in which the accumulator step size is maintained constant is known as linear delta modulation. For many signals (e.g., telephone speech), the linear delta modulation process requires unreasonably high bit rates to avoid excessive slope overload distortion. This disadvantage has been overcome by the development of "adaptive delta modulation" in which the accumulator step size is dynamically adjusted in accordance with the slope of the source signal, or in accordance with this slope as inferred from observation of the digital signal generated by the comparator. Transmission quality of an adaptive delta modulation channel is comparable to that of a pulse code modulation system operating at the same bit rate. In general, multi-channel delta modulation communication systems employ an encoder and a decoder for each channel. The digital outputs of the encoders are multiplexed for transmission and demultiplexed upon reception and applied to individual decoders. In my copending application Ser. No. 444,822, filed Feb. 22, 1974, U.S. Pat. No. 3,949,298, there is described a multi-channel delta modulation system providing communication between a plurality of analog signal sources and corresponding signal sinks employing a time shared delta modulation encoder and decoder.
Still another method which has been suggested for converting an analog signal to digital form is differential pulse code modulation (DPCM). In DPCM the source signal is sampled at the same rate as in PCM, but the quantity which is encoded and transmitted digitally to the receiving terminal is the difference between the current analog input sample and the quantized approximation of the last sample. In DPCM, the coder generates a multibit code word for each difference, whereas delta modulation provides only one code bit for each difference signal. The DPCM coder may be a linear coder in which the coding step sizes are uniform or may be a non-linear coder in which the step size depends on the difference amplitude. In either case, the smallest step size should be small for good accuracy but the coding range of the coder should be large enough to accommodate the expected range of the difference signals. At the transmitter, the encoded quantized difference signal increments a local accumulator so that the accumulator output is the sum of all previous increments. The accumulator output is a close approximation to the analog input signal. At the receiver, a similar accumulator is incremented by the same encoded quantized difference signal and provides a representation of the analog signal.
A DPCM system in which the coder-decoder step size is invariant with time provides only a small improvement in channel quality compared to conventional PCM operating at the same bit rate. However, a DPCM system can be made "adaptive" by automatically adjusting the coder-decoder step size according to the results of previous encodings. Adaptive differential PCM (ADPCM) provides channel quality comparable to conventional PCM but at substantially lower bit rates. Quantative comparisons for single channel ADPCM have been reported by Cummiskey, Jayant, and Flanagan in a paper entitled "Adaptive Quantization in Differential PCM Coding of Speech," published in volume 52 of the Bell System Technical Journal (September 1973, pp 1105-1118).
The coders and decoders used for DPCM and adaptive DPCM are relatively complex. Consequently, in the multi-channel systems presently under consideration where one coder and one decoder are employed for each analog signal, the systems become relatively expensive, and as a consequence DPCM and ADPCM have not been used in multi-channel communication systems.